The invention relates to creating and modifying curves on a computer display.
Curves in graphics programs, such as the Illustrator program from Adobe Systems Incorporated in San Jose, Calif., can be created on a computer display by a user using free-hand motion of a mouse. Generally, such curves are represented as constructs of control points. For example, for a third-order Bezier curve segment (as shown in FIG. 12), four control points are used in which two control points P0 and P3 are segment end points (through which the curve segment passes), and the two control points P1 and P2 are slope points defining the slope of the curve segment at the segment end points P0 and P3.
A composite curve can be formed by concatenating multiple Bezier curve segments. In some prior systems, such as in the Adobe Illustrator system, a user can modify a composite curve by selecting and dragging control points. The grabbed points then move by the distance specified by the cursor. In other systems, such as that discussed in U.S. Pat. No. 5,412,770, a point on a curve can be fixed and a segment of the curve can be reshaped by moving a cursor on a computer display. In another prior system, a composite curve can be modified by dragging a control point after fixed points are specified on the composite curve.